The changes in tumor pO2, known to occur after irradiation [O'Hara, et al., Rad.Res.144:222, 1995] may result from alterations of tumor perfusion and/or oxygen consumption by the tumor cells. In this study we have determined the effects of 10 Gy X-irradiation, given in vivo, upon the rates of oxygen consumption of cells harvested from two different transplantable murine tumors, RIF-1 and MTG-B, that exhibit different kinetics of radiation-induced hypoxia and reoxygenation. These data were compared with matched cells from non-irradiated tumors, and also with cells of the same tumor type irradiated as monolayers in vitro. Single cell preparations are suspended in 10% dextran in medium, and O2 consumption determined by the EPR linewidth measurement of a neutral nitroxide. For comparison between experiments, the experimental values are expressed as a ratio of the rate of O2 consumption by irradiated cells to unirradiated controls on the same day. The rate of O2 consumption by cells derived from irradiated tumors at the time of the lowest observed tumor pO2 (6 hr for MTG-B, 24 hr for RIF-1) was approximately twice that of unirradiated controls. Cultured MTG-B cells showed transient increased O2 consumption 2-6 hr post-irradiation, while 24 hr post-irradiation this rate was less than that of unirradiated cells (p<.05, n=4). These results support the hypothesis that post-irradiation changes in tumor pO2 can result, at least in part, from radiation-induced changes in oxygen consumption by tumor cells.